The law and the police service differ across the UK, so pick your nation for the correct guidance.
Some law is UK-wide and applies in all three nations: the Online Safety Act 2023 communications offences and Ofcom’s platform powers, and the call-recording position under the Investigatory Powers Act 2016. But harassment, stalking, and threats offences differ by nation, so the victim selects their nation for the correct statutes and police service (a Home Office force in England & Wales, Police Scotland, or the PSNI in Northern Ireland).